A stable isotope tracer technique was used to investigate the loss rate of methyl bromide in surface ocean waters. Unfiltered and 0.2 μm-filtered or autoclaved aliquants of Biscayne Bay seawater samples were spiked with 13CH3Br at roughly 10--100 times ambient concentrations(50--800 pM) and incubated for 10--30 hours. The concentration of13CH3Br was monitored using gas chromatography with isotope dilution mass spectrometry, withCD3Br as the isotope spike. Removal rates in unfiltered aliquants were significantly faster than in the 0.2 μm-filtered or autoclaved aliquants, indicating that some of the loss of methyl bromide was associated with particulate matter. Filtration experiments indicate that the particulate material responsible for methyl bromide loss is between 0.2 and 1.2 μm in diameter, suggesting that bacteria are likely to be responsible. The particulate-related removal of methyl bromide was inhibited by autoclaving, supporting a biological mechanism.¿ 1997 American Geophysical Union